CHAPTER 1. MEMORIES ARE MADE OF THIS
HOW MEMORIES ARE FORMED
THE BRAIN CONSTANTLY SIMMERS WITH ACTIVITY AS
DIFFERENT GROUPS OF NEURONS, DRIVING OUR DIFFERENT
THOUGHTS OR PERCEPTIONS, DRIFT IN AND OUT
OF ACTION LIKE FIREFLIES IN A FOREST.
A memory is the reactivation
of a specific group of
neurons. But what allows
a specific combination of neurons
to be reactivated over any other
combination of neurons?
The answer is synaptic
plasticity. This term
describes the persistent
changes in the strength
of connections – called
synapses – between brain
cells. These connections
can be made stronger
or weaker depending on
when and how often they
have been activated in the
past. Active connections tend to
get stronger, whereas those that aren’t
used get weaker and can eventually
disappear entirely. This gives added
meaning to the term use it or lose it!
Not many features of the brain are more
important than synaptic plasticity.
Changing the strength of existing
synapses, or even adding new ones or
removing old ones, is critical to memory
formation. But there is also evidence
that another type of plasticity, not
directly involving synapses, could
be important for memory
formation. In some parts of
the adult brain, such as
the important memory
structure known as the
hippocampus, brand
new neurons can be
created in a process
called neurogenesis.
Studies in older mice
have shown that by
increasing neurogenesis in
the hippocampus, memory can be
improved. In humans, exercise has been
shown to increase the volume of the
hippocampus – suggesting new neurons
are being created – and at the same time
improve performance in memory tasks.
Memory Pathways
A GOOD ANALOGY for memory formation
is the way foot traffic creates a path along a
stretch of grass. The more a patch of grass
is trampled as people pass along it, the
clearer the path becomes and the easier
it is to follow – it’s as if a ‘memory’ of all
the walking has been created. The same
thing happens in the brain. The more a
neural pathway is activated, the stronger
the synaptic connections along the way
become. Then, when a thought enters our
head – say, a tropical beach – we recall
related experiences or knowledge, such as
putting on sunscreen and the feel of sand, as
our minds funnel our thoughts along well-
established neural pathways.